A couple reasons I would guess:

1. Full carrying cost of an employee is much more then their salary so this math is not as straight forward if you’re just cutting time and salary to account for that time.

2. You should assume most people aren’t counting hours in places like Meta, reducing to a 4 day week imho will start making people think more about counting exact hours they’re working. It’s partially why the “4 10s” concept is also a bad idea that permeates the defense contractors.

3. Staying focused 5 days a week for one person probably has better compounding effects for that week than a few people working part time and taking longer to get the work done with longer breaks in between “sessions”. Harder to measure of course but it’s one thing I’d be worried about. Easier to think about if you say each person works 2.5 days a week for half their pay, I’d rather just have one person.

4. Layoffs let you cut by performance.